In photography, a bracketing technique refers to a process of capturing a sequence of images that can be used, e.g., to generate an HDR image. Such a sequence of images can be referred to as a bracket. An example of a conventional bracket is represented as follows:[n(EV−),EV0,n(EV+)],                where EV0 refers to an image that is captured using an ideal exposure value (EV) given the lighting conditions at hand,        where n refers to a number of stops above or below the EV0 image,        where n(EV−) refers to an underexposed image that is captured at n lower stops than the EV0 image, and        where n(EV+) refers to an overexposed image that is captured at n higher stops than the EV0 image.        
In the conventional bracket shown above, “EV” stands for exposure value and refers to a given exposure level for an image (which may be controlled by one or more settings of a device, such as an imaging device's shutter speed and/or aperture setting). Different images may be captured at different EVs, with a one EV difference (also known as a “stop”) between images equating to a predefined power difference in exposure. Typically, a stop is used to denote a power of two difference between exposures. Thus, changing the exposure value can change an amount of light received for a given image, depending on whether the EV is increased or decreased. For example, one stop doubles or halves the amount of light received for a given image, depending on whether the EV is increased or decreased, respectively.
The “EV0” image in a conventional bracket refers to an image that is captured using an exposure value as determined by an imaging device's exposure algorithm. Generally, the EV0 image is assumed to have the ideal exposure value (EV) given the lighting conditions at hand. It is to be understood that the use of the term “ideal” in the context of the EV0 image herein refers to an ideal exposure value, as calculated for a given image capture system. In other words, it is a system-relevant version of ideal exposure. Different image capture systems may have different versions of ideal exposure values for given lighting conditions and/or may utilize different constraints and analyses to determine exposure settings for the capture of an EV0 image. The “EV−” image refers to an underexposed image that is captured at a lower stop (e.g., 0.5, 1, 2, or 3 stops) than the EV0 image. For example, a “1EV−” image refers to an underexposed image that is captured at one stop below the exposure value of the EV0 image. The “EV+” image refers to an overexposed image that is captured at a higher stop (e.g., 0.5, 1, 2, or 3) than the EV0 image. For example, a “2EV+” image refers to an overexposed image that is captured at two stops above the exposure value of the EV0 image.
A common approach to HDR image creation comprises: (i) sequentially capturing a bracket with different exposure settings (e.g., the EV0 image, the EV− image, the EV+ image, etc.); (ii) performing image registration on the bracket's images; (iii) performing image fusion on the registered images; and (iv) tone mapping the fused images. For the image registration and image fusion techniques, one conventional technique is to set the EV0 image as the reference image and then register, warp, and fuse other images (e.g., the EV− image, the EV+ image, etc.) to the EV0 image. One technique of image fusion in HDR imaging techniques is to replace pixels in the EV0 image having incomplete information (e.g., clipped pixels, etc.) with corresponding pixels from the EV− image that have complete information to recover some or all of the lost pixel information. This technique, however, may be suboptimal because it always assumes that pixels in the EV0 image having incomplete information will always have corresponding pixels from the EV− image that have complete information. In practice, this assumption is not always true. This is because, in some scenarios, pixels in the EV0 image that have incomplete information might not have corresponding pixels from the EV− image that have complete information due to motion and occlusion. In these scenarios, there are no pixels with complete information that can be used to replace or recover the pixels in the EV0 image having incomplete information. When image registration and image fusion techniques are performed on the bracket using the incomplete EV0 image as the reference image, the resulting HDR image may include one or more artifacts. Pixels affected by unwanted artifacts are also referred to herein as artifact pixels.
To reduce or eliminate artifacts, the EV− image (instead of the EV0 image) may be set as the reference image and then the other images in the bracket may be registered, warped, and fused with the EV− image. Here, the EV− image is used as the reference image based on an assumption that pixels from the EV− image have a lower likelihood of being pixels with incomplete information than corresponding pixels from the EV0 image. One limitation of this technique is that the EV− image is much noisier than the EV0 image, and as a result, the HDR image will be much noisier than an HDR image formed using the EV0 image as the reference image.